We associate the pink ribbons with Faith, Goodman Brown's young wife, whose innocence and devout character are seemed to be represented in part by the colour of her ribbons. I think potentially that the falling to the ground of these ribbons can symbolise one of the themes of the story, which is the fact that we are all sinners - none of us are so good as to avoid the taint of sin on our lives entirely. It is clear that at the beginning of the story Goodman Brown idolises his wife, and yet the 'dream' he has in the forest causes him to doubt his impressions of her. It is this 'fall' from the pedestal that he has put her on that is symbolised by the fall of the ribbons.
Don't forget though that pink is not pure innocence such as the color white would suggest. Pink is created by mixing white with red. Red has symbolic associations with blood, love, passion, evil, fire, etc. While red can have both positive and negative suggestions, the red fire and the evil of the devil are clearly brought up in this story, and Faith's pink ribbons are found, and abondoned at the witch meeting in the dark forest. Whatever innocence they may have represented is lost, and they never represented pure innocence at all. Faith is tempted and fearful at the start of the story and wants Brown to stay with her, perhaps to keep her from the temptation he thinks is only his challenge.
Pink is a color that is symbolic of innocence and naivete. In Hawthorne's "Young Goodman Brown," the pink ribbons that Faith wears at the beginning of the narrative represent the ingenuousness of Brown, who does not understand the terrible significance of the journey upon which he embarks with the old man.
As he is in the forest and witnesses the Black Sabbath, Brown sees the pink ribbons of Faith fall to the ground, indicating Brown's personal loss of faith.